When blocking a user, and leaving the talk page editable (the default for nearly all blocks to allow for appeals), the user talk page is not watch by default.
Generally, it would be desirable for the blocking user to watch that page, as otherwise they will not be informed automatically about an appeal of a block that they set (unless the blocked user knows how to ping, which is certainly not a given).
So I suggest the "watch the users page" option to default to checked.